This invention relates broadly to developments concerning equipment for electrical measurements on conductors. The invention will be described herein with reference to fault indicators for power distribution cables, it will be appreciated, however, that the invention does have broader applications, including for example in stand alone current measurements on electrical conductors.
Equipment for detection and location of faults on power lines involve typically the measurement of the magnetic fields produced by the alternating current in power lines, using a single magnetic field sensing coil.
In substations, this involves expensive current transformers, which must also provide insulation between the power line conductor and earth potential.
There is also equipment which can be located throughout a network which does not provide its own insulation between the phase and earth potential because it is mounted either at phase potential or at earth potential.
Such equipment normally derives a signal proportional to the average or peak magnetic field by rectifying the waveform to produce a DC voltage and this is used for the detection and location of faults. By deriving a signal proportional to the average or peak magnetic field, other waveform parameters, phase, and harmonic content information is removed from the signal. DC voltages are suitable for analogue amplification, for operating analogue control devices and for interpretation by analogue comparators.
When the power lines experience a fault, high currents flow in the conductors of the lines, producing a rapid increase in the magnetic fields around the conductors. Therefore, if an increase is detected by the equipment, this is indicative of a fault current having passed the magnetic field sensing coil. Typically, within the equipment a derivative of the output signal of the magnetic field sensing coil is produced in an analogue electronic circuit, to detect increases in the amplitude of current flow. The currents flowing in the conductors of the power lines, particularly during a fault situation, may typically vary between 5 to 25,000 Amperes. The average magnetic field around the power line conductors therefore has a high dynamic range, which is typically between 1 to 10,000.
The types of coil which may be used to measure magnetic fields in such equipment are i) air-cored coils which are typically cylindrical, ii) air-cored toroids, iii) coils, cylindrical or toroidal, which are cored with a ferromagnetic or paramagnetic medium other than air. Each type of coil has specific advantages and disadvantages. Air-cored coils do not saturate in the presence of high magnetic fields and can therefore be used to detect magnetic fields with a high dynamic range. However, with air-cored coils, particularly cylindrical coils mounted at earth potential some distance from the power line conductor, low magnetic fields generate only low induced signals in the coil and may therefore be difficult to detect accurately, in particularly when background signals may contribute to the measurements.,
On the other hand, coils cored with a para- or ferromagnetic medium, increase the induced signal in the coil due to the high permeability which results in an increase of the magnetic field inside the coil due to magnetic polarisation of the medium. However, such coils have the disadvantage of saturating once the xe2x80x9ctruexe2x80x9d magnetic field to be measured exceeds a particular value, and therefore the characteristics of fields in excess of that particular value cannot be measured with such coils.
In the equipment for detection and location of faults on power lines, the average magnetic field signal derived from the coils is processed using an analogue circuit. Some fault detectors may employ an analogue variable gain control utilising for example a variable resistor such as a Junction Field Effect Transistor (JFET) to increase the dynamic range for the measurement, however, due to the analog nature of such circuits the output signal is then not directly representative of measured magnetic field strength. In some fault detectors, such as those described in U.S. Pat. No. 4,947,126 and U.S. Pat. No. 5,270,898, a gain may be employed which is switched between a high and low value by analogue circuitry utilizing for example, switches or relays. However, due to the analogue nature of such circuits, a dynamic range of greater than 16:1 is difficult to attain and scaling of the amplitude is normally lost. The amplitude of the signal in the sensing coil and that of the output signal lose their one-to-one relationship and this cannot be restored using analogue circuitry alone.
The equipment described above has the limitation of providing little information on the magnetic field, waveform, phase or harmonic content and electric field waveform, phase or harmonic content preceding, during and after the fault, since the information utilised is substantially limited to the identification of sudden changes in the amplitude of the magnetic field detected by the magnetic field sensing coil. The further information about the magnetic field and electric field preceding, during and after the fault can be useful in determining the characteristics of the fault. These characteristics include the severity of the fault (ratio of fault current to pre-fault current), the time and duration of the fault current and the time for the protection equipment to operate, whether the fault was phase to phase or phase to earth, and whether the fault current was accompanied by a fuse or circuit breaker operation (loss of voltage) or a substantial variation in the voltage.
Furthermore, the equipment described above is not able to discriminate between a fault current and what is referred to as magnetising inrush currents, which are typically observed when voltage is applied to a non-faulted power system following an extended outage period. Therefore, when the equipment detects loss of system voltage, it usually inhibits its detection of faults until a predetermined period after voltage is reapplied, resulting in a period during which fault detection for the power line concerned is impossible.
In accordance with a first aspect of the present invention there is provided a method for measuring at least one characteristic parameter of an alternating current in a conductor, the method comprising the steps of:
measuring the magnetic field around the conductor at a point along the conductor;
deriving an analogue voltage signal representative of the measured magnetic field;
adding a direct current (DC) offset signal to an alternating current (AC) component of the measured magnetic field;
amplifying the analogue voltage signal;
converting the amplified voltage signal into a digital voltage signal;
measuring the digital voltage signal and, when the amplitude of digital voltage signal reaches a predetermined value, adjusting a gain setting of the amplification; and
generating an output signal representative of the parameter of the alternating current based on the amplified voltage signal and the gain setting.
The step of adding a DC offset signal allows for making the measured magnetic field suitable for amplification while substantially maintaining waveform information.
Accordingly, in at least preferred embodiments of the present invention, a large dynamic range can be realised for the magnetic field/alternating current measurements. Preferably, the method can be used in conjunction with an air-cored coil not to be limited by a saturation effect.
The characteristic parameter of the alternating current may be one of the group of waveform characteristics of the alternating current such as amplitude of the alternating current; frequency of the alternating current; phase of the alternating current; harmonic content of the alternating current; and a derivative of the alternating current. More than one parameters may be measured simultaneously.
The method may preferably further comprise the step of integrating the analogue voltage signal for obtaining the waveform of the alternating current in the conductor.
Thereby, in at least preferred embodiments of the present invention, even low magnetic field strengths may be measured at high main settings.
In one embodiment, the step of adding a DC offset signal comprises the step of varying the DC offset signal in response to a DC component in the digital voltage signal.
In a preferred embodiment, the method further comprises the step of digitally filtering the digital voltage signal to determine the ratio of a mains voltage signal at a mains frequency of the conductor and an harmonic voltage signal at an harmonic of the mains frequency. In one embodiment, a second harmonic is utilised for determining whether a magnetic field signal around the conductor is caused by magnetising inrush current.
Preferably, the method further comprises the step of measuring an electric field in the vicinity of the conductor; and deriving a second analogue voltage signal representative of the measured electric field in the vicinity of the conductor.
In one embodiment, the step of measuring the electric field comprises the step of placing a capacitor arrangement in the vicinity of the conductor; and the method further comprises the steps of amplifying the second analog voltage signal; converting the amplified second voltage signal into a second digital voltage signal; measuring the second digital voltage signal and, if the amplitude of the second digital voltage signal reaches a second predetermined value, adjusting a second gain setting of the second amplification; and generating a second output signal representative of at least one parameter of a voltage signal in the conductor derived from the second amplified voltage signal and the second gain setting.
The characteristic parameter of the voltage signal may be one of the group of waveform characteristics of the voltage signal such as amplitude of the voltage signal; frequency of the voltage signal; phase of the voltage signal; and harmonic content of the voltage signal. More than one parameters may be measured simultaneously.
In accordance with a second aspect of the present invention there is provided an apparatus for measuring at least one characteristic parameter of an alternating current in a conductor, the apparatus comprising:
measuring means for measuring the magnetic field around the conductor at a point along the conductor;
means for deriving an analogue voltage signal representative of the measured magnetic field;
trimming means for providing a DC offset signal to an AC component of the measured magnetic field;
amplification means for amplifying the analogue voltage signal;
converting means for converting the amplified voltage signal into a digital voltage signal;
gain control means for adjusting a gain setting of the amplification depending on the amplitude of the digital voltage signal; and
means for generating an output signal representative of the parameter of the alternating current based on the amplified voltage signal and the gain setting.
The trimming means allows for making the measured magnetic field suitable for amplification while substantially maintaining waveform information.
The characteristic parameter of the alternating current may be one of the group of waveform characteristic of the alternating current such as amplitude of the alternating current; frequency of the alternating current; phase of the alternating current; and harmonic content of the alternating current. More than one parameters may be measured simultaneously.
The apparatus may preferably further comprise means for integrating the analogue voltage signal to obtain the waveform of the alternating current in the conductor.
In one embodiment, the apparatus further comprises filtering means for filtering the digital signal and means for determining the ratio of a mains voltage signal at a mains frequency of the conductor and an harmonic voltage signal at an harmonic of the mains frequency.
Preferably, the apparatus further comprises a second measuring means for measuring an electric field in the vicinity of the conductor; and means for deriving a second analogue voltage signal corresponding to the measured electric field in the vicinity of the conductor.
In one embodiment, the apparatus further comprises second amplification means for amplifying the second analog voltage signal; second converting means for converting the second amplified voltage signal into a second digital voltage signal; second gain control means for adjusting a second gain setting of the second amplification means depending on the second digital voltage signal; and a second means for generating a second output signal representative of at least one parameter of a voltage signal in the conductor derived from the second amplified voltage signal and the second gain setting.
The characteristic parameter of the voltage signal may be one of the group of waveform characteristics of the voltage signal such as amplitude of the voltage signal; frequency of the voltage signal; phase content of the voltage signal; and harmonic content of the voltage signal. More than one parameters may be measured simultaneously.
The amplification, conversion, gain control and output may be performed by the same respective components for both the electric and magnetic field measurements in the apparatus.
In one embodiment, the amplification means comprises first and second operational amplifiers in series, wherein the analogue voltage signal is applied to the non-inverting input of the first operational amplifier, and the output of the first operational amplifier is connected to the non-inverting input of the second operational amplifier.
In one embodiment, the gain control means comprises a digitally controlled analogue switch having a low xe2x80x9coffxe2x80x9d resistance and a high xe2x80x9conxe2x80x9d resistance.
Preferably, the trimming means comprises a serial to parallel shift register, wherein the outputs of the shift register are connected to a resister network to form a digital to analogue converter.
The present invention may be more readily understood from the description of preferred forms of an apparatus for electrical measurements on conductors given below with reference to the accompanying drawings.